[LINUX] Allows userspace tools on a domU to set up a watch on the xenstore.

[LINUX] Allows userspace tools on a domU to set up a watch on the xenstore.
It does this by intercepting XS_WATCH requests written
to /proc/xen/xenbus and then re-submitting the request to the
in-kernel xenstore interface, in
linux-2.6-xen-sparse/drivers/xen/xenbus/xenbus_xs.c.  When a callback
occurs, an in-kernel function is invoked, which then reconstructs a
response in the format expected by userspace, and sends this response
through /proc/xen/xenbus.

It was necessary to add some supporting infrastructure to
linux-2.6-xen-sparse/drivers/xen/xenbus/xenbus_dev.c, such as an
additional mutex to protect the response queue and a list of active
watches associated with each connection.

Signed-off-by: Michael LeMay <mdlemay@epoch.ncsc.mil>